Flexible cable shielding

ABSTRACT

In a shielded multiple-conductor cable, a shield comprising parallel, longitudinal, convoluted wires symmetrically disposed around the conductor bundle, where the convoluted shield shape is maintained by means of inner and outer tie wraps.

United States Patent Madle 5] May 28, 1972 [54] FLEXIBLE CABLE SHIELDING3,351,706 11/1967 Gnerre et a] ..l74/l08 x 3,328,510 6/1967 White..174/1 15 X lnventofl Madle, Torrance, Cahf- 3,163,711 12/1964Schindler... ..174 116 Assigneez The states of a Peterson 1 Xrepresented y the of the Air 2,140,270 POttfil' "174/115 X Force1,745,096 1/1930 Jayne ..174/115 x Filed: y 1971 Primary Examiner-Lewis1-1. Myers Assistant Examiner-A. T. Grimley [21 1 Appl' 140175AttorneyHarry A. Herbert, Jr. and Henry 5. Miller, Jr

52 U.S.Cl ..174/107,174/27, 174/36, 57 ABSTRACT 174 108,174 ll5,l74lOFC, 174 ll0S,l74ll0 H l l 1 /AR In a shlelded muluple-conductor cable,a shleld compnsmg 51 1m. (:1. ..H01b 7/18 Parallel kmgimdinal, wiressymmerical'y 58 Field 61 Search I ..174/107 36 102 R 110 PC dispmd ammdbundle Where 174/1 10 10 AR H5 H7 M 27 10's 3 R shield shape ismaintained by means of inner and outer tie wraps. [56] Cited 8 Claims, 1Drawing figure UNITED STATES PATENTS 3,607,490 9/1971 '11 inzm nn,,..,...,..174 113 Patented May 23, 1972 .1 I'I'I'IAZ'III'L JINVENTOK zfiw?ATTORNEYS FLEXIBLE CABLE SI'IIELDING BACKGROUND OF THE INVENTION Thisinvention relates generally to electrical cable shielding and moreparticularly to an improved flexible electrical cable shielding. Largemissiles have a need for multiple-conductor cables following complexthree-dimensional routes and having connectors at the ends of manybranches. These cables must provide a high degree of isolation orshielding between the conductors and outside environment.

Heretofore, such shielding has usually been obtained either through theuse of convoluted metallic pipe or by an overbraid of woven wires. Theseprior art devices are limited in their ability to provide torsionalflexibility, thereby causing difficulty where more than one area ofdesign is considered. Thus if a shielded cable is designed for ease ofinstallation then experience has shown that the cable may be prone toexcessive mechanical failure due to repeated torsional stress.

One of the major limitations found in the overbraid type of shielding isthe inflexibility caused by the requirements for several layers oftightly woven wire merely to meet the shielding requirements of missileuse. In order to attain an idealized cable, several factors must beconsidered. The overall dominating factor is the shielding effect of thecable. Shielding is determined by considering the skin depth. A skindepth is defined as that distance below the surface of a conductor wherethe current density has diminished to e" of its value at the surface.

The shielding attenuation achieved by a homogeneous conductive sheetsurrounding the conductor bundle, is a function of the thickness of theconductive sheet expressed in the number of skin depths.

A second consideration taken with regard to the shielding factor is themagnetic coupling that occurs between the current in the shield materialand the conductor bundle whenever the shield current is not perfectlysymmetrically disposed around the periphery of the shield. Similarly, acapacitative coupling occurs between the shield-current carrying pathand the conductor bundle whenever the shield-current passes through. aseries impedance, regardless of the symmetry or lack thereof.Frequently, mechanically joined surfaces in series with the shieldcurrent path having milli or microhm impedances can cause significantenergy coupling'due solely to the surface contact resistance of metalsso joined even when steps have been taken to avoid this situation. Sincecables are generally long and relatively thin and are equipped withheavy connectors at multiple branch-ends and must be installed onmissiles in contorted and confined spaces, certain mechanical propertiesare of prime importance in providing an improved cable. These mechanicalproperties include bending flexibility, torsional flexibility,extensibility, tear strength, kind kink and abrasion resistance. If acable cannot provide these characteristics, it would lack the propertiesneeded for missiles and would not be an improvement over the prior art.7

Possibly the most difficult problem to compensate for in establishing animproved shielded cable is that of fatigue or work hardening caused bythe low frequency, large amplitude portions of the vibration and shockenvironment. A cable structure would then, have to be inherentlyrelatively inextensible under vibration or shock induced tension.

SUMMARY OF THE INVENTION The shield of the invention utilizes a largenumber of longitudinal, uninsulated, untwisted,parallel, wires extendingthe length of a cable assembly and symmetrically disposed around theperiphery of the conductor bundle. The invention minimizes thelongitudinal impedance of the shield wires themselves as well aseliminates all longitudinal joints except those between the wires andthe connector backshells at the ends of the cable.

The longitudinal wires are so arranged around the conductor bundle thatthey touch each other thereby allowing the shield current to distributeitself symmetrically around the periphery of the cable. Multiple layersof longitudinal wires could be used to vary the thickness of the shieldand hence the shielding attenuation factor.

By placing an overcovering around the outside of the shield theinvention has relatively high torsional flexibility, in that theovercovering will allow for a slight sliding movement between adjacentwires. To improve the bending flexibility the parallel bundle of wiresare convoluted in a manner analogous to that used for convoluted pipesor bellows. The wires are convoluted by the use of tie wraps used underand over each layer of wire causing it to assume a sine wave shape alongits longitudinal axis. To provide length stability, the conductor bundleis layedup in a contra-helical fashion such that tensile forces tend toproduce equal and opposing torsions resulting in longitudinal stiffness.

It is therefore an object to provide a new and improved cable shieldthat will increase the overall reliability of shielded cables.

It is another object of the invention to provide a new and improvedcable shield that has improved stability of performance duringinstallation, transportation and flight compared with those presently inexistence.

It is a further object of the invention to provide a new and improvedcable shield that will be economical to produce.

It is still another object of the invention to provide a new andimproved cable shielding that is mechanically independent of theshielding design factors.

It is still a further object of the invention to provide a shieldedcable that is more flexible than any hitherto known.

It is another object of the invention to provide an electrical cableshield that is relatively uneffected by magnetic and capacitativecoupling.

It is another object of the invention to provide an electricallyshielded cable that is resistant to kinking and snagging.

It is another object of the invention to provide an electrical cablethat has a low degree of linear extensibility.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiment in the accompanying drawing.

DESCRIPTION OF THE DRAWING The FIGURE is a side elevational view partlyin section of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the FIGURE, theelectrical cable and shielding are shown generally at 10. The cableconductor bundle is shown generally at 12 and is formed from strandedcopper conductors 14 insulated with a low coefiicient of frictionresilient insulation 16, for example Teflon, to allow the relativesliding motions necessary for both bending and tor- SlOll.

The inner jacket 18 consists of a relatively loose Teflon spiral wraphaving two layers, overlapped and having no bond therebetween. Thislayer provides good electrical insulation between the conductor bundleand the shield under severe external loads, such as crushing or localimpact, while not restricting the sliding motions necessary for bendingor torston.

The inner tie wraps 20 are placed in a wide spaced spiral wrap and areformed of a continuous rubber or rubber like hollow pipe. The inner tiewrap is placed directly upon the inner jacket 18.

The shield 22 is symmetrical disposition of wires around the innerjacket. Where the conductor bundle has a diameter of 1 inch, forexample, approximately 1,000 ends of number 30 AWG bare conductiveshield wire would be sufiicient for most shielding purposes. This wouldbe the equivalent of three wires radial thickness all around a 1 inchdiameter conductor bundle. The outer tie wrap 24 is a wide spaced spiralwrap of hollow rubber pipe identical with that utilized for the innertie wrap 20. This wrap should be of the same hand and centered betweenturns of the inner tie wrap. The wrap is applied over the shield wires22 with minimum tension so as to avoid either flattening or ovaling ofeither itself or the inner tie wrap.

The inner and outer tie wraps lightly grip the shield wires at intervalsalong the cable length when the cable is straight. As the cable is bentthe wraps flatten or oval" thereby allowing the shield wires on theoutside of the cable bend to effectively lengthen. The shield wiresremain free to slide relative to each other, in the portions of theirlengths between the inner and outer tie wraps, when the cable is undertorsional force.

The outer jacket 26 is abrasion resistant and highly resillent I and maybe formed from at least two overlapped and bonded layers of siliconrubber tape and adhesive. The outer jacket may be increased in thicknessby the addition of wrap layers as necessary for thermal protection ofthe shield and conductor bundle.

As may be clearly seen, the mechanical and shielding properties of suchconstruction are interrelated but the general design is such that asufficient number of details may be varied to meet a broad range ofrequirements for bending flexibility, torsional flexibility, shieldingattenuation, thermal insulation and abrasion resistance.

I claim:

l. A shielded multiple-conductor cablecomprising: a plurality ofinsulated wires forming a conductor bundle and wound in a contra-helicaldirection; a first layer of insulative material wrapped around theconductive bundle; a first tie wrap, wound in a spaced spiral around theconductor bundle and overlying the insulative layer; a plurality ofclosely spaced, uninsulated, untwisted, parallel wires extending alongthe iongitudinal axis of the cable overlying the first tie wrap; asecond tie wrap, wound in a spaced spiral around the wires andpositioned alternately of the first tie wrap; and a second layer ofabrasion resistant insulative material providing a covering for theshield and conductor bundle.

2. A shielded multiple-conductor cable comprising:

a plurality of insulated wires forming a conductor bundle and wound in acontra-helical direction;

a first layer of insulative material wrapped around the conductivebundle;

a first tie wrap wound in a spaced spiral around the conductor bundleand overlying the insulative layer; and

a plurality of closely spaced, uninsulated wires extending along thelongitudinal axis of the cable overlying the first tie wrap, wound in aspaced spiral around the wires and positioned alternately of the firsttie wrap.

3. A cable as claimed in claim 2 wherein said plurality of closelyspaced wires are untwisted.

4. A cable as claimed in claim 2 wherein said plurality of closelyspaced wires are parallel.

5. A cable as claimed in claim 2 wherein said plurality of closelyspaced wires are untwisted and parallel.

6. A cable as claimed in claim 2 and further including:

a second layer of abrasion resistant insulative material providing acovering for the shield and conductor bundle.

7. A cable as claimed in claim 2 and further including:

a second layer of thermally insulating material providing a covering forthe shield and conductor bundle.

8. A cable as claimed in claim 2 and further including:

a second layer of thermally insulating and abrasion resistant insulativematerial providing a covering for the shield and conductor bundle.

1. A shielded multiple-conductor cable comprising: a plurality ofinsulated wires forming a conductor bundle and wound in a contra-helicaldirection; a first layer of insulative material wrapped around theconductive bundle; a first tie wrap, wound in a spaced spiral around theconductor bundle and overlying the insulative layer; a plurality ofclosely spaced, uninsulated, untwisted, parallel wires extending alongthe longitudinal axis of the cable overlying the first tie wrap; asecond tie wrap, wound in a spaced spiral around the wires andpositioned alternately of the first tie wrap; and a second layer ofabrasion resistant insulative material providing a covering for theshield and conductor bundle.
 2. A shielded multiple-conductor cablecomprising: a plurality of insulated wires forming a conductor bundleand wound in a contra-helical direction; a first layer of insulativematerial wrapped around the conductive bundle; a first tie wrap wound ina spaced spiral around the conductor bundle and overlying the insulativelayer; and a plurality of closely spaced, uninsulated wires extendingalong the longitudinal axis of the cable overlying the first tie wrap,wound in a spaced spiral around the wires and positioned alternately ofthe first tie wrap.
 3. A cable as claimed in claim 2 wherein saidplurality of closely spaced wires are untwisted.
 4. A cable as claimedin claim 2 wherein said plurality of closely spaced wires are parallel.5. A cable as claimed in claim 2 wherein said plurality of closelyspaced wires are untwisted and parallel.
 6. A cable as claimed in claim2 and further including: a second layer of abrasion resistant insulativemateRial providing a covering for the shield and conductor bundle.
 7. Acable as claimed in claim 2 and further including: a second layer ofthermally insulating material providing a covering for the shield andconductor bundle.
 8. A cable as claimed in claim 2 and furtherincluding: a second layer of thermally insulating and abrasion resistantinsulative material providing a covering for the shield and conductorbundle.